Photographs of
normal and mutant mice showing variations of eyelids at birth and wound
healing as adults.

PROTEIN
CRITICAL FOR DEVELOPMENT IN FRUIT FLIES FOUND
TO AID HEALING OF CUTS AND WOUNDS IN MAMMALS

Biologists at the University
of California, San Diego have determined that a protein essential for
the normal embryonic development of fruit flies is also used by mammals
to assist in the timely healing of cuts and lacerations.

Their discovery, detailed in
the June 3 issue of the journal Developmental Cell, provides
new insight for scientists into the molecular mechanisms responsible for
wound healing in humans and may one day lead to the design of new drugs
for individuals whose healing is compromised.

Improving
wound healing is particularly important for burn victims and others with
slow healing skin lesions like those associated with diabetes. Diabetes
now affects more than 17 million Americans and the most prevalent form—type
2, or insulin-resistant diabetes—continues to skyrocket. The U.S.
Centers for Disease Control and Prevention estimate that the number of
diagnosed cases will increase 165 percent by 2050, as the number of overweight
adults and children climbs in the United States.

Geneticists have long been aware
of the role played by “c-Jun”—a protein called a “transcription
factor” because it turns genes on and off— in bringing cells
together during the embryonic development of Drosophila, the
fruit flies commonly used in genetic research. For many years, Drosophila
biologists have even suspected that the same biochemical pathways involved
in a process called dorsal closure in the development of fly embryos were
involved in wound healing in mammals. During dorsal closure in Drosophila,
sheets of cells come together and fuse along the dorsal midline. The process
looks similar to the way sheets of skin cells come together during wound
healing in mammals.

“However, healing skin
is a very different thing than putting together sheets of cells in making
a fly,” says Randall S. Johnson, an associate professor of biology
at UCSD who headed the research team. “So we really weren’t
sure what to expect when we began our study. But we eventually demonstrated
in our experiments that the loss of this protein in the skin of mice causes
cells to bunch up at the leading edge of a wound, much like water rushing
into a curb.”

“If we can eventually
design drugs to promote this chemical pathway,” he adds, “surgeons
will be able to improve the recovery of their patients and the increasing
number of diabetics in this country will be able to improve their ability
to heal from cuts and lacerations.”

The researchers in the Johnson
lab were assisted by Kit Pogliano, an associate professor of biology at
USCD; Steven K. Hanks of Vanderbilt University School of Medicine; Katie
Nason and Jeffrey M. Arbeit from UC San Francisco School of Medicine and
Ronald M. Wisdom from UC Davis.

While geneticists previously
knew that in Drosophila c-Jun is needed to initiate the movement
of cells in dorsal closure, a major difference between wound healing in
mammals and dorsal closure in Drosophila is that, in wound healing,
skin cells that were together become separated as a result of injury.
In dorsal closure in Drosophila, cells that were never together
receive some signal that makes them migrate towards each other. To see
if c-Jun played a role in wound healing in mammals, Cindy Gustafson-Brown
and Guochun Li, postdoctoral fellows in Johnson’s lab, produced
a strain of mice that lacked the c-Jun protein in skin cells.

Surprisingly, these mice were
born with open eyes. Mice, like kittens and puppies, are usually born
with closed eyes. Eyelid closure occurs in the later stages of embryonic
development when skin cells rapidly proliferate and migrate across the
eye. Initially the researchers were quite puzzled by the open-eye defect
in the mice.

“We spent a long time scratching
our heads and wondering, what does that mean?” says Johnson. “It
was when we also saw the pattern of migration in the wound that it became
clearer.”

The wounds of mice lacking c-Jun
protein in skin cells healed more slowly than the wounds of wild-type
mice. Even more revealing were microscopic images of skin samples near
the wound. Wound healing is a multi-stage process that begins with the
formation of a fibrin clot, the initial scab, and ends with the migration
and proliferation of skin cells through the fibrin clot to close the wound.
However, in the mice lacking c-Jun, skin cells seemed to proliferate properly,
but bunched up at the edge of the wound and failed to move across.

“The clear malformation
at the leading edge indicates that this is the target zone for the function
of this protein during wound healing,” says Johnson.

The researchers also found that
the skin cells at the edge of the eye in mutant mice were bunching up
and stalling, just as they did at the edge of the wound. To understand
the molecular reason for these defects, the researchers took skin tissue
samples from the wound and the edge of the eye in mutant and normal mice,
then stained them with markers to certain proteins known to be involved
in cell communication in Drosophila. They found decreased levels
of several of these proteins in the leading edge skin cells along the
wound and eye in the mutant mice. The cells from the mutant mice also
had defects in the cytoskeleton, the cell’s support structure, which
is important for cell mobility.

The protein c-Jun is a member
of a family of proteins in which many proteins have overlapping functions.
This may explain why the mutant mice did not have more severe skin defects,
says Li, the first author of the study. “C-jun might be specifically
required when the balance of skin cell proliferation and differentiation
are rapidly and temporally altered, such as in wound healing or growth
of tumors.”

In addition to the implications
of these findings for drug design to promote wound healing, Li points
out, “Our mutant mice give us a terrific tool to study the function
of this protein in tumor formation and in other skin diseases.”